Standard x-ray radiography allows an x-ray image of a target object to be obtained, by capturing x-rays transmitted through the object onto a film screen. The x-ray image is a projection of the object along the x-ray direction. In the x-ray image, variations in the composition and thickness of the target object in different regions of the object are revealed, as a result of their differential absorption of the x-rays.
Digital radiography allows x-ray images to be captured digitally. Typically, an array of digital detectors and a digital image processor are used. Digital radiography systems allow the radiologist to window the image, i.e. to adjust the image's contrast. In this way, subtle details, which may be missed by the fixed, limited contrast in traditional film-screen radiography, can be detected. Thus, digital x-ray systems produce images having a greatly enhanced contrast, compared to the contrast in the images obtained by traditional film-screen radiography. the resolution of digital x-ray images is also greatly enhanced compared to the resolution of images obtained using other techniques, for example CT scanning.
There is no depth information, however, in the images obtained by stationary two-dimensional arrays of digital detectors, unlike in the images obtained by CT scanning. Digital x-ray systems produce 2-D images of the object, as projected along the x-ray direction and onto a lateral plane parallel to the plane formed by the detectors. These 2-D images have no resolution in the longitudinal dimension parallel to the x-ray beam and orthogonal to the detector plane formed by the detectors.
Digital tomosynthesis enables a three-dimensional (3-D) image of an object to be constructed from a finite set of 2-D projection images of the object, by acquiring multiple 2-D images from multiple angles, and then reconstituting the data. Using digital tomosynthesis, a certain degree of resolution can be produced in the longitudinal dimension. Typically, in digital tomosynthesis systems the x-ray source is rotated during data acquisition, for example in an arc through a limited angular range, and a set of projection radiographs of the object are acquired by a stationary detector at discrete locations of the x-ray source. In many systems, the digital x-ray detector array and the object are maintained in a stationary or fixed position, while the x-ray source is moved to multiple locations in order to obtain a collection of images from different view angles.
The multiple 2-D images are then combined or synthesized, using known digital averaging and filtering techniques. Each synthesized image is focused on a tomography plane, which is parallel to the detector plane, and which is located at a certain depth along the longitudinal direction. In a simplest method, the 2-D projection images are spatially translated with respect to each other, and superimposed in such a way that the images overlap precisely in the tomography plane. Other methods for reconstructing tomographic images may use sophisticated computation techniques similar to those used in computerized tomography (CT) systems. The location of the tomography plane within the object can be varied, and a 2-D image of the object can be obtained for each location of the tomography plane.
One of the drawbacks of digital tomosynthesis is that the relative location of the x-ray source with respect to the detector array must be known with great precision, as the x-ray source is being moved with respect to the object being imaged. In the process of synthesizing the images, it is necessary to align the collected images with a precision equal to, or better than, the lateral resolution. It is difficult, however, to move the x-ray source to multiple positions with the requisite degree of precision. Uncertainties in the position of the x-ray source and/or the x-ray detector generally contribute to image blurring.
Accordingly, it is desirable to provide a high-precision image positioning method and system for digital tomosynthesis that allows the position of the x-ray source and the object to be determined within a desired resolution, without requiring precise physical positioning of the x-ray source at the desired resolution. Such high-precision image positioning method and system would greatly reduce the mechanical requirement of the imaging system for tomosynthesis.